1. Field of the Invention
The present invention relates to an automatic focusing circuit and more particularly, to an improvement of an automatic focusing circuit for automatically matching the focus in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera and an electronic still camera having an automatic focusing mechanism.
2. Description of the Prior Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as a video camera and an electronic still camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating the state in which the focus is controlled has been developed. According to such an approach, many good characteristics can be obtained. For example, there exists substantially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
As an example of such a focus control method utilizing a video signal, a control method referred to as a so-called hill-climbing servo system has been conventionally known. The hill-climbing servo system is described in, for example, U.S. Pat. Nos. 4,638,364, 4,614,975, Japanese Patent Laying-Open Gazettes Nos. 58505/1983 and 103776/1985. Briefly stated, a high frequency component of a video signal obtained from an image sensor is detected every one field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens continues to be slightly vibrated so that the focus evaluating value always takes the maximal value.
FIG. 1 is a schematic block diagram showing an example of an automatic focusing circuit for a conventional video camera utilizing such a hill-climbing servo system, and FIG. 2 is a block diagram showing the details of a focus evaluating value generating circuit 5 shown in FIG. 1.
Referring to FIGS. 1 and 2, description is made on a conventional automatic focusing circuit using a hill-climbing servo system.
Referring to FIG. 1, a video camera comprises a focusing ring 2 for moving a focusing lens 1, a focusing motor 3 for driving the focusing ring 2, and an image sensing circuit 4 including an image sensor (not shown) such as a CCD (Charge Coupled Device). The focusing lens 1 may be moved by a piezoelectric element instead of a motor. In addition, the image sensor itself (not shown) such as the CCD instead of the focusing lens may be moved by the piezoelectric element.
An image formed on a surface of the image sensor by the focusing lens 1 is converted into a video signal by the image sensing circuit 4 and inputted to the focus evaluating value generating circuit 5. Referring to FIG. 2 showing the details of the focus evaluating value generating circuit 5, a luminance signal component in a video signal outputted from the image sensing circuit 4 is applied to a synchronizing separator circuit 5a and a gate circuit 5c. The synchronizing separator circuit 5a separates a vertical synchronizing signal VD and a horizontal synchronizing signal HD from the inputted luminance signal and applies the same to a gate control circuit 5b. The gate control circuit 5b sets a rectangular sampling area in a central portion of a picture in response to the inputted vertical synchronizing signal VD and horizontal synchronizing signal HD and a fixed output of an oscillator (not shown). The gate control circuit 5b applies a signal for opening or closing a gate every field to the gate circuit 5c so that passage of the luminance signal is permitted only in the range of the sampling area. The gate circuit 5c may be provided anywhere in the former stage of an integration circuit 5f as described below.
Only the luminance signal corresponding to the range of the sampling area is applied to a high-pass filter 5d every field by the gate circuit 5c. The high frequency component of the video signal separated by the high-pass filter 5d is amplitude-detected by a detector 5e, the detected output being applied to the integration circuit 5f. The integration circuit 5f integrates every field the detected output applied thereto, the integrated output being applied to an A/D converter 5g. The A/D converter 5g converts the integrated value inputted thereto into a digital value and supplies the digital value as a focus evaluating value in the current field. The supplied focus evaluating value is applied to a first memory 6 as described below.
Returning to FIG. 1, a focus evaluating value outputted from a focus evaluating value generating circuit 5 is stored in the first memory 6. When a focus evaluating value in the next field is then outputted from the focus evaluating value generating circuit 5, data stored in the first memory 6 is transferred to a second memory 7. More specifically, the contents of the first memory 6 and the second memory 7 are updated every field, so that the newest focus evaluating value and a focus evaluating value one field before are always stored in the first memory 6 and the second memory 7, respectively. The contents of the two memories 6 and 7 are inputted to a comparator 8 and compared therein. The compared output is applied to a focusing motor control circuit 9.
As a result of comparison by the comparator 8, when the focus evaluating value stored in the first memory 6 is larger than that stored in the second memory 7, the focus evaluating value is increasing, so that the focusing motor control circuit 9 maintains the current rotational direction of the focusing motor 3 in response to an output of the comparator 8. On the other hand, when the focus evaluating value stored in the first memory 6 is smaller than that stored in the second memory 7, the focus evaluating value is decreasing, so that the focusing motor control circuit 9 reverses the rotational direction of the focusing motor 3 in response to the output of the comparator 8. The focusing ring 2 supporting the focusing lens 1 continues to move in the direction of increasing a focus evaluating value by such movement of the focusing motor 3, so that the in-focus state is achieved. After achieving the in-focus state, the focusing ring 2 and the focusing lens 1 continue to be vibrated back and forth in the vicinity of the maximal point of the focus evaluating value.
In the above described hill-climbing servo system, if only the slope of a focus evaluating value is detected, the focusing lens 1 is not stopped in the defocused position by driving the focusing lens 1 in the direction of always increasing the focus evaluating value even if the object is changed, so that very good follow-up characteristics can be achieved.
However, such a hill-climbing servo system suffers from the following significant disadvantages caused by continuing to vibrate the position of the focusing lens.
A first disadvantage is that since the focusing lens is not stopped even in the in-focus state, a picture continues to be vibrated even if an object at rest is in focus. For example, the focal length of a lens currently used in a television camera is changed by rotating a focusing ring, so that the angle of field of a sensed image is changed. Therefore, in the above described system in which the focusing ring continues to be vibrated even in the in-focus state, an object on the picture becomes large or small with a particular cycle, resulting in a very unclear picture.
A second disadvantage is directed to a power consumption. There are many cases where a home video camera currently utilizes a battery as a power supply due to the portability thereof. When a focusing motor is always driven as in the above described hill-climbing servo system so that the forward rotation and the reverse rotation are repeated, more power is consumed, as compared with when the focusing motor is rotated in a constant direction, due to in-rush current, so that the time period during which an image can be recorded by using such a battery becomes short.
Additionally, since the focusing ring is always rotated, a problem of wear of a gear occurs, for example.
In order to overcome these disadvantages, there is proposed a system for detecting the maximal point where a focus evaluating value is changed from an increasing tendency to a decreasing tendency by driving a focusing ring in a one-way direction, and returning the focusing ring to the maximal point and stopping the same therein, which is disclosed in Japanese Utility Model Laying-Open Gazette No. 135712/1985. In detecting the maximal value, focus evaluating values are compared every one field, the larger focus evaluating value is always stored as the maximum value, and the maximum value is determined as the maximal value when it is determined that the current focus evaluating value has dropped, by a predetermined threshold value, from the maximum value.
On the other hand, in a video camera, the position of the focus must be changed, following an object which changes momentarily. Even after the lens is once stopped in the in-focus position as described above, a hill-climbing operation of the lens must be resumed when the distance between the object and the lens is changed. Therefore, an approach of determining that an object changed when the focus evaluating value changed, by more than a predetermined threshold value, while the focusing lens is stopped and of resuming the hill-climbing operation is proposed by one of the inventors of the present invention, which is disclosed in Japanese Patent Application No. 252545 filed Nov. 11, 1985. According to this approach, the position of the focus can be changed following an object which changes momentarily. On the other hand, this approach suffers from disadvantages as described below.
More specifically, when an object to be sensed is in focus, if the other object crosses an area between the video camera and the object to be sensed and more particularly, a field of view (referred to as an automatic focusing area hereinafter) corresponding to the above described sampling area set in a picture, then the lens is displaced such that the other object is brought into focus. As a result, an image of the original object is defocused and the angle of field is changed, resulting in a very unclear image. FIG. 3A is a diagram showing diagrammatically such a case. Referring to FIG. 3A, it is assumed that a video camera Ca is recording an original object P in an in-focus state and the other object Q is out of an automatic focusing area AF which is a field of view corresponding to the above described sampling area. When the other object Q enters the automatic focusing area AF and passes in front of the camera Ca as represented by an arrow in FIG. 3A, all or a part of the original object P seen from the camera Ca is obstructed by the other object. Consequently, a hill-climbing operation is resumed such that the other object Q is brought into focus, so that the lens begins to move. However, since some time periods are required until the object Q is in focus, the object Q has passed through the automatic focusing area AF before movement of the lens is completed. As a result, while the object Q is passing through the automatic focusing area, the object P is out of focus and the object Q is not in focus. In addition, immediately after passage of the object Q is completed, the original object P is on the picture in the defocused state, resulting in a very unclear picture.
Conventionally, in the case of the transition from the in-focus state to the defocused state, an approach of always prohibiting driving of the lens during a constant time period, for example, the time period expected to be required until the object Q has passed through the automatic focusing area AF has been proposed. According to this approach, the hill-climbing operation is not resumed in the case of transient passage as shown in FIG. 3A. On the other hand, if the other object Q enters the automatic focusing area AF and then remains in this area while the object P is recorded as shown in FIG. 3C, the hill-climbing operation is resumed such that the object Q is brought into focus after a constant time period.
However, according to this approach, if the original object P itself approaches the camera as represented by an arrow in FIG. 3B (or leaves the camera), it is not until the above described constant time period is elapsed after the defocused state is determined that driving of the lens is resumed, so that follow-up characteristics of the focus relative to the object is decreased and response characteristics of the automatic focusing operation is decreased.